PSI - Issue 33

Lorenzo Vigna et al. / Procedia Structural Integrity 33 (2021) 623–629 Author name / Structural Integrity Procedia 00 (2019) 000–000

627 5

The main failure mechanism is delamination with formation of two fronds (Fig. 2b) and some powders due to matrix fragmentation. Different failure modes are found after the test, as shown in Fig. 2b: some specimens show higher permanent deformation causing a wider opening angle of the two fronds, whereas other specimens show a stronger elastic return of the external layers that causes a narrower opening angle of the fronds. The presence of different failure modes could be the reason for the scatter in the experimental results. To assess the influence of the failure mode on the SEA, the opening angle of the specimen fronds (‘fracture angle’ in Fig. 4a) has been measured and considered in the analysis of the experimental data. The SEA measured in each test is then plotted as a function of the fracture angle (Fig. 4b). A good correlation between SEA and fracture angle is found, probably due to the higher level of deformation of the material, that causes a higher energy absorption. Considering the failure mode as a new parameter in the analysis, it is then possible to correct the results to hide the effect of the failure mode and reveal the effect of the investigated factors (i.e., impact velocity and clamping force). If the values of SEA are modified to get to an ideal situation where the failure angles are the same for all the specimens (an average angle of 120° in this case), this corresponds to the translation of all the experimental points in Fig. 4b parallel to the regression line in Fig. 4b that interpolates them according to the following formula: ���� � � ������ � � , (2) where k = 0.131 kJ/(kg°) is the slope of the regression line in Fig. 4b, is the experimental SEA value and is the fracture angle. With this correction, results change as shown in Fig. 5; the scatter is reduced and, even though no effect of the impact velocity is visible in Fig. 5a, the increase of the corrected SEA with the clamping force is clearly visible in Fig. 5b. The friction due to the sliding contact between the specimen and the supporting columns is then influent and must be controlled carefully to avoid an overestimation of the measured SEA. For this material, an increase of 1 kJ/(kg  kN) is found.

a

b

Fig. 4. a) Measurement of the fracture angle of a crashed specimen. b) SEA as a function of the fracture angle.